The present invention relates to a steering device suitably used in fork lift trucks and other industrial vehicles.
It is known in the art that a steering device for use on fork lift trucks, constructional and agricultural and other industrial vehicles, comprises: a steering shaft which has a steering wheel fixed to its upper end and is rotatably supported at its lower end by a gearbox secured to a chassis frame of the vehicle; a steering column of tubular configuration rotatably supporting and enclosing the steering shaft with an annular spacing or radial gap between the outside diameter of the shaft and the inside diameter of the column; and a column clamp fixed to the chassis frame and holding the steering column at a longitudinally upper portion thereof. Although this type of steering device is simple in structure and sufficiently rigid, it suffers shortcomings that the steering wheel vibrates and the vibration makes a vehicle operator uncomfortable. These shortcomings are aggravated particularly when the vehicle uses a diesel engine as a drive source.
To solve the above shortcomings, the conventional types of steering devices were investigated and examined in detail through both experimental and theoretical approaches, and the following fact was found evident.
There are two causes for vibration of a steering wheel. One is a resonance between the steering device and a vehicle engine, and the other is a forced vibration, i.e., an oscillating movement transmitted from a chassis frame to the steering device. In more detail, engines vibrate at a frequency which is a multiple of the number of revolutions (rpm) thereof. Specifically, a four-cylinder engine vibrates at the greatest magnitude at a frequency level which is two times the engine revolution. The steering device vibrates in resonance with the vibration of the engine at such frequency when the vibrating frequency of the engine coincides with a natural frequency of the steering device. Since a conventional steering device has a point of resonance within a normal running speed range of an engine (e.g. 600 to 2,400 rpm), the steering device starts to oscillate when the running speed or revolution of the engine reaches a level corresponding to the specific resonance point. On the other hand, as the steering device is connected to the vehicle chassis frame via a gearbox and a steering column clamp, the oscillatory movements of the chassis frame are transmitted to the steering device through those two connection points or members.
In view of the above analysis, prevention of the vibration of the steering wheel requires that the resonance point of the steering device be shifted out of the normal running speed range of the engine. To shift the resonance frequency of the steering wheel, it is effective to reduce the rigidity of connection of the steering device with the chassis frame. The reduction of the above-mentioned rigidity may produce the other cause for vibration, i.e., a forced vibration (an oscillating movement transmitted from a chassis frame to the steering device). In order to reduce the forced vibration, it is necessary to change the shape of vibration mode of the steering device in which the amplitude of the vibration is small at the position of the steering wheel. Furthermore, a mere reduction of the rigidity of connection of the steering device with the chassis frame will cause the steering wheel to be easily moved in the radial directions and thereby reduce not only an ease of operation thereof by an operator but also its capability of supporting the operator's weight when the vehicle is started, stopped or turned. Therefore, it is required to construct the steering device such that the transmission of the vibration from the chassis frame to the steering device is effectively shut off while the above problem of reduced operationability and weight supporting capability of the steering wheel is overcome at the same time.